amiserial: remove serial_state::xmit_fifo_size

It's always 1, so define a macro for it instead. Note that the check in
set_serial_info is doubled, hence remove the latter.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210714091314.8292-4-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>